Method for texturing a metallic thin film

ABSTRACT

Method for texturing a surface of a metallic thin film of a magnetic disk substrate comprising the steps of: (a) frictionally contacting the surface of the metallic thin film with an abrasive article, wherein the abrasive article comprises an abrasive coating including a water-soluble binder and abrasive particles, wherein the abrasive coating is adhesively attached to a flexible foraminous fibrous backing; and (b) abrading the surface of the metallic thin film by moving the metallic thin film and the abrasive article relative to one another with the abrasive coating in contact with an aqueous liquid solution to form scratches in the surface of the metallic thin film, wherein the liquid solution is free of abrasive particles before contact is initiated with the abrasive coating.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for texturing a metallic thinfilm with an abrasive article, wherein the abrasive article comprises anerodible abrasive coating attached to a foraminous fibrous backing.

2. Description of the Related Art

Personal computers and their usage have become pervasive in contemporarytimes. Many personal computers contain a rigid memory disk or harddrive. A hard drive involves a rigid thin film metal-coated disk ornonmetal disk as the substrate of the magnetic medium. In oneconventional arrangement, the thin film rigid disks are manufactured byelectroless nickel plating a thin-film of nickel or nickel alloy onto analuminum base, such as forming nickel/phosphorus (Ni-P) coating on thealuminum base. The Ni-P coating is then polished to a very fine,mirror-like finish. After polishing, the Ni-P coating is textured,followed by the application of a magnetic coating(s) thereon to form themagnetic medium.

The texturing portion of this process is critical to the ultimateperformance of the rigid disks. The texturing process preferably resultsin a random pattern of uniform scratches with sharply defined edges in asubstantially circumferential direction relative to the center of therigid disk.

Texturing accomplishes a number of purposes. It improves theaerodynamics between the computer head (which reads and writes data onthe disk) and the thin film rigid disk as the disk spins beneath thehead. It also improves the magnetic properties of the coated disks. Thescratches formed during texturing make it easier for the head todistinguish bytes of information between tracks on the disk. Thetexturing also preserves the separation between the computer head andthe rigid disk when the computer is first turned on. When the computeris turned on and energized, the rigid disk will begin to spin. If thedisk is smooth and untextured, this head/disk contact makes it onerousfor a disk to start spinning. This phenomenon is referred to asstiction/friction in the argot of the computer industry.

To provide texturing which can deliver these and other advantages, it isimperative that the profile and topography of the texturing scratchesformed in the surface of the thin film are carefully managed. Forinstance, if the scratches formed are too deep there may be a potentialloss of data on the rigid disk. Also, it is important that the surfaceroughness imparted into the textured surface is relatively uniformacross the width of the surface. That is, the undulations of thescratches should be as regular in pitch as possible.

Also, the portion of the metal thin film portion of the substrateabraded away during texturing is known in the industry as swarf.Practice has shown that excess swarf generated during the use of somelapping films is still apt to be present at the interface of theabrasive coating and the substrate work surfaces. Therefore, thereremains some opportunity for the swarf to become attached to form highspots on the textured rigid substrate where conventional lapping filmsare employed. That particular phenomenon is known in the industry asreweld. Those high spots are highly undesirable as they can collide withthe computer head during use, which can cause a loss of data and/or headdamage as a result of the collision.

Prior to the present invention, the texturing process for thin metalfilms of magnetic disks was traditionally accomplished by using a looseabrasive slurry. The loose abrasive slurries provide substantiallycircumferential scratches that have sharply defined edges having therequisite depth. Loose abrasive slurries are, however, accompanied by anumber of disadvantages. These include the inconvenience of handling therequired volume of the slurry, the required agitation to preventsettling of the abrasive granules and to assure a uniform concentrationof abrasive granules at the grinding interface, and the need foradditional equipment to prepare, handle and also recover and recycle theabrasive slurry. Additionally, the loose slurry itself must be analyzedto assure its quality and dispersion stability requiring additionalcostly man hours. Furthermore, pump heads, valves, feed lines, grindinglaps, and other parts of the slurry supply equipment which contact theslurry show undesirable wear. Further yet, the loose abrasive slurriesare untidy; creating a large amount of debris and waste in and about thevicinity of the texturing operation. As a result, the thin film rigiddisks, after texturing, must be thoroughly cleaned to remove anyresidues left on their surface from the abrasive slurry.

Not surprisingly, to overcome the numerous disadvantages associated withloose abrasive slurries, integral coated abrasive lapping films havebeen used to texture the thin film rigid disks. An example of such aproduct is "IMPERIAL" Lapping film (Type R3) commercially available from3M Company, St. Paul, Minn. This lapping film comprises a polymeric filmbacking having an abrasive coating layer bonded thereto. The abrasivecoating layer includes very fine abrasive particles (average particlesize less than 10 micrometers) dispersed in a binder that is coated onthe polymeric film and solidified to form a thin abrasive coating layer(about 10-15 micrometers). The surface profile of the abrasive coatingis essentially flat other than the partial protrusions formed of some ofthe fine abrasive particles. During use, the lapping film abrades aportion of the substrate surface, thereby texturing the surface of thesubstrate. Similarly, U.S. Pat. No. 4,974,373 to Kawashima et al.describes an abrasive tool suited for use in lapping, polishing,texturing, and various other finishes of precision machine parts,mentioning hard disks and magnetic heads, as well as ceramics, plastics,and jewels, involving abrasive powder particles fixed in a separatedproximity to each other in a binder resin coat disposed on a plasticfilm base to form the abrasive tool. However, in addition to problemswith reweld, conventional lapping films may not provide scratches havingedges as sharp and/or clean as those produced by the loose abrasiveslurries. These lower quality scratch edges may degrade the quality ofthe disks manufactured using lapping film for the texturing process.

As a recent alternate proposal to use of such lapping films fortexturing, the use of porous nonwoven cloths coated on a surface with anabrasive layer has been advanced as another method to uniformly texturethin film metal or metal alloy coated rigid disks before application ofthe magnetic coatings in a clean process that generates high qualityscratches and avoids the problem of reweld. For example, U.S. Pat. No.5,307,593 (Lucker et al.) discloses a nonwoven substrate coated with anabrasive layer that is used in a method for texturing magnetic mediasubstrates having a thin-film metal or metal alloy coating, where theporous nonwoven substrate provides advantages such as the ability tocollect and entrap the swarf and debris during the abrasion procedureaway from the work interface, among other things. Lucker et al. employ awater-insoluble binder in the abrasive layer used to texture a thin-filmmetal.

U.S. Pat. No. 5,236,762 (Suzuki et al.), corresponding to EuropeanPatent Application No. 0 438 671, discloses an abrasive film suitablefor use in the finishing of magnetic heads, magnetic disks, micrometers,watches, molds and so forth, comprising a film substrate having anabrasive layer on at least one side thereof, wherein the abrasive layercontains abradant particles unifromly dispersed in at least one binderselected from water-soluble macromolecular substances andwater-dispersible macromolecular substances. However, the filmsubstrates disclosed in Suzuki et al. are resinous nonfibrous films. Acontinuous resinous film, inherently, would have no porosity and wouldaggravate loading problems on the working side of the abrasive article.

What is desired in the field of rigid disk texturing is an abrasiveproduct with the convenience of a coated abrasive product that producesresults similar to that obtained with a loose abrasive slurry whileavoiding the the aforementioned disadvantages associated with looseabrasive slurries.

In general, the provision of erodible abrasive films to simulate a looseslurry has been proposed. For example, U.S. Pat. No. 4,255,164 (Butzkeet al.) teaches an abrasive article for use in fining ophthalmic lenses.The article comprises a flexible backing sheet and a brittlemicrocellular coating formed of water-insoluble modified phenol or ureaformaldehyde resinous binding material, which will disintegrate duringuse, creating an abrasive slurry.

Also, U.S. Pat. No. 4,576,612 (Shukla et al.) teaches a polishing padrestricted to polishing glass or plastic ophthalmic lenses. Thepolishing pad of Shukla et al. is described as including a flexiblesubstrate and a flexible matrix coated on said substrate involvingpolishing particles contained in a binder matrix composed of a latexmaterial and a water soluble polymer. Shukla et al. refers to usage ofwater soluble polymers alone as the abrasive particle binder, presumablyfor glass polishing operations, sans the latex (viz. acrylic latex)component. However, Shukla et al. characterized the outcome for thatapparent glass polishing operation as being poor.

U.S. Pat. No. 5,104,421 (Takizawa et al.) teaches a polishing padcomprising a substrate coated with a blend of abrasives and awater-soluble cellulose ether binder. Takizawa et al. also describeconventional sheet-like abrasives of sand papers and polishing tapesmanufactued by bonding abrasive grains or particles on the faces ofpaper or fabric sheet-like substrates through synthetic water-solublehigh molecular compounds such as polyvinyl alcohol or natural substancessuch as gelatin. However, Takizawa et al. indicate that suchconventional polishing tapes manufactured by using such water-solublebonds have no waterproofness and poor bonding strength resulting inswift falling grains from the faces of sand paper.

JP 5-228845, published 7 September 1993, teaches a polishing film fortexturing of magnetic disk substrates, comprising a polymeric film andan abrasive layer comprising abrasive particles and a water solubleresin, where the abrasive particles are released from the film substrateduring use in an aqueous environment.

SUMMARY OF THE INVENTION

The present invention provides a method for texturing a metallic thinfilm of a magnetic disk with a self-slurrying, foraminous-backedabrasive sheet article to provide a regular surface finish therein withreduction of self-welding problems. The method of the present inventionalso minimizes the loading problems attendant to nonporous-backedpolishing films and the liquid handling steps, untidiness and equipmentcosts associated with the use of conventional loose abrasive slurries.

In one embodiment, the method of the present invention for texturing asurface of a metallic thin film of a rigid magnetic disk substratecomprises frictionally contacting the surface of the metallic thin filmwith an abrasive article comprising an abrasive coating on a flexibleforaminous fibrous backing, wherein the abrasive coating comprises awater-soluble binder which releasably bonds therein fine abrasiveparticles; and then abrading the surface of the metallic thin film bymoving the metallic thin film and the abrasive article relative to oneanother with the abrasive coating in contact with an aqueous liquidsolution to form scratches in the surface of the metallic thin film,where the liquid solution is free of abrasive particles as supplied fromits source up until contact with the abrasive coating is initiated.

A small amount of water only solution, or other aqueous liquid which issupplied as an abrasive particle-free solution, is brought into contactwith the surface of the abrasive coating of the abrasive article used inthe method of the invention, as a prewetting solution, preferably,and/or during the abrading procedure itself, to permit the sheet tocreate its own slurry in situ during the texturing operation, whileallowing the user to start with a clean, easily handled dry sheet. Theabrasive sheet used in the method of the present invention includes acoating which constitutes a dry abrasive slurry bonded to a backingwhich erodes during use and gradually forms an effective loose abrasiveslurry capable of texturing metallic thin films at least as well asconventional loose slurries. The abrasive coating composition isprepared, coated, and dried to yield a coating which will erode ordisintegrate and release its loading of abrasive particles at acontrolled rate under use conditions. The gradually released abrasiveparticles are capable of rotating freely and therby generate a uniformlyscratched surface.

That is, the clean and uniform grooves are achieved by prewetting theabrasive constructions so that the water-soluble binder will dissolvecreating a pseudo slurry for texturing the disk. This prewetting of theabrasive construction can be done approximately 2-3 cm in front of thetexturing interface so as to ensure that the binder is suitablydissolved.

In a further embodiment, the abrasive article is useful for texturingnickel and nickel alloy plated (thin film) rigid memory disks. Thenickel coating, such as Ni-P, can be formed on a rigid metal baseselected from metal, glass or ceramic material. The magnetic disksubstrate comprises a rigid base material having opposing majorsurfaces, the metallic thin film, such as Ni-P, formed on at least oneof the major surfaces. In one preferred embodiment, the abrasive articleused in the method of the invention includes a nonwoven backing with abarrier coating, where the nonwoven backing is slurry coated with adispersion of water soluble resin and abrasive particles.

The preferred water soluble binders include polyvinyl alcohol,polyacrylamide, or polyethylene glycol, singly or in any combinationsthereof. The fibrous backings used in the abrasive article of the methodof the invention preferably include fibers comprising cotton, polyester,and their blends as woven or nonwoven (paper-like) materials. Thepreferred backing is a foraminous (porous) polyester fiber nonwovenmaterial. In one embodiment, the abrasive coating comprises the abrasiveparticles and the water-soluble binder in a weight ratio of 7:1 to 3:1,respectively. The abrasive particles generally have an average particlesize of about 0.05 to about 5 micrometers, preferably between 0.1 to 3micrometers. In one preferred embodiment, the abrasive particlescomprise aluminum oxide, such as white aluminum oxide.

In another further embodiment of the inventive method, the abrasivearticle comprises a sheet configuration frictionally conveyed over thesurface of the metallic thin film by reel-to-reel means. In general, theabrading step forms substantially circumferential scratches in thesurface of the metallic thin film with an Ra of between about20angstroms to about 70 angstroms, preferably an Ra between about 30angstroms to about 50 angstroms. In another embodiment, the rigidmagnetic disk substrate comprises a circular shape and a central axis,and further wherein the step of abrading further comprises rotating therigid substrate about the central axis to form substantiallycircumferential scratches. Further, the step of rotating furthercomprises forming the circumferential scratches at a speed of at least7.5 meters per minute at an interface between the metallic thin film andthe abrasive article. In yet another embodiment, The step of abradingfurther comprises oscillating the abrasive article in a directionsubstantially perpendicular to a direction of travel of the rigidsubstrate during the abrading. The method of the invention alsocontemplates the additional step of forming at least one magnetic layeron the textured (abraded) surface of the metallic thin film.

Other features, advantages, and constructs of the invention will bebetter understood from the the following description of the figures andthe preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view of a thin-film metal coated rigid disk substratetextured according to the method of the present invention.

FIG. 2 is a cross-sectional view of a textured thin-film metal coateddisk substrate produced by the method of the present invention takenalong direction 2--2 of FIG. 1.

FIG. 3 is a cross-sectional view of an abrasive article used in themethod of the present invention.

FIG. 4 is a schematic of a texturing apparatus for use with the methodof the present invention.

FIG. 5 is a line plot of the surface finish Ra measured across thesurface of disks textured with an abrasive article of the method of thepresent invention and a Comparative Example.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides a method for texturing thin-film metal ormetal alloy coated rigid disks with an abrasive article comprising anabrasive coating, in which abrasive particles are dispersed in a watersoluble binder, coated on a surface of a foraminous fibrous backingsheet. It is preferred that the abrasive particles be uniformlydispersed throughout the water soluble binder in the abrasive coating.In this form, as the abrasive article is used under wet conditions, thewater soluble binder softens and releases abrasive particles, therebyexposing fresh abrasive particles still held in the undissolved binder.It is this combination of the water soluble binder and a porous fibrousbacking that results in an improved texturing operation on a thin-filmmetal layer of a magnetic substarte. The water soluble binder allows fora relatively fast erodibility and the porous backing allows a means toremove or pull the swarf and used abrasive particles away from thepolishing interface. Thus, while not desiring to be bound to any theoryat this time, it nonetheless is believed that during polishing, thatonly fresh abrasive particles are presented at the interface of the worksurface and abrasive sheet article during polishing. Shortly thereafter,the worn abrasive particles are expelled and removed from the polishinginterface. Additionally, it is theorized that the water used duringpolishing softens the water soluble binder and as a result a loose,thick paste of abrasive particles and water soluble binder is formed.This paste, or loose abrasive slurry then polishes the workpiecesurface. The abrasive particles have more limited movement than withslurries, and hence produce surface finishes uniquely suited to thinfilm disks.

In one embodiment, the process of texturing a magnetic medium disksubstrate using the method of the present invention includes providing arigid substrate that has a thickness between 0.75 to 1.25 millimeter.The magnetic disk substrates include a thin-film metal layer formed on arigid metal or nonmetal material. The disk substrate preferably involvesa thin-film metal or metal alloy coating applied onto a metal base,where the metal base is preferably an aluminum alloy. however, thenonmetal materials usable as substrate for the thin-film metalpreferably are glass or ceramic. As can be understood, the "magneticsubstrate" of the invention means a thin film member constituted byplural distinct superposed layers, which is susceptible to applicationand adherence of a magnetic layer thereon. The "rigid magnetic disksubstrate" means a generally circular disk constituted of the metallicthin film and a rigid base layer formed into an integral article, whichis susceptible to application and adherance of a magnetic layer thereon.

Referring to the rigid substrate for a memory disk as illustrated inFIG. 1, the rigid substrate disk 10 is generally circular in shape withcenter hole 11. A thin-film metal or metal alloy coating 13 is appliedover at least one surface 12 of a disc-shaped metal base 25 (shown inFIG. 2). The thin-film metal coating is typically applied to bothopposite major surfaces of the rigid substrate disk 10.

For purposes of this invention, the "metallic thin film" coatingincludes metal or metal alloy coatings. The metal is typically appliedby electroless nickel plating, although other coating techniques may beemployed. The metallic thin film coating thickness generally is in arange between 5 to 20 micrometers, and more typically is about 15micrometers. The preferred nickel coating includes phosphorous toprevent the nickel from having magnetic properties. The preferredcoating typically contains from about 5 to 20% phosphorous, usuallyabout 12% phosphorous. The preferred metal coating does not contain anyappreciable amounts of metal oxides, i.e., the amount of metal oxides byweight are typically less than 0.01%.

After metal coating, but before texturing, the rigid disk substratesurface 12 can be polished to a very fine finish, usually by aconventional loose abrasive slurry. Loose abrasive slurries comprise aplurality of abrasive particles (typically having an average particlesize less than 5 micrometers) dispersed in a liquid medium, such aswater or an organic solvent. After polishing with the loose abrasiveslurry, the metal coating has a very fine random scratch pattern ororientation.

The arithmetic average of all distances from the centerline of theroughness profile of the surface of the polished metal coatingpreferably has a value of less than 20 angstroms. That surface finishvalue is referred to herein as Ra, and is also known as Center LineAverage. As referred to in connection with the present invention, Ra ismeasured using a Wyko TOPO-3D Interferometer (purchased from Wyko Corp.,Tucson, Ark.) with a 40X objective lens. It will be understood thatother methods of measuring Ra could be used in connection with themethod of the present invention, with appropriate adjustments to thepreferred values of Ra as discussed herein.

After polishing, the metal coating on surface 12 is ready to be texturedaccording to the method of the present invention. Texturing of the metalcoating on the surface 12 results in a random pattern of scratches 14 ina substantially circumferential direction relative to the center of therigid disk substrate 10. The scratches are preferably nonconcentric withthe center of the rigid disk substrate 10, but preferably substantiallycircumferential, producing scratches that randomly cross each other.

Referring now to the partial cross-sectional view of FIG. 2, the rigidsubstrate disk 10 comprises metal base 25 with a textured metal coating13 formed on both substrate surfaces 12 and 22, although it will beunderstood that the coating could be present on only one major surface12. Scratches 14 are irregular in nature and comprise high regions 24and low regions 26. The Ra of the scratches 14 is typically betweenabout 20 angstroms (0.0020 micrometers) and about 70 angstroms (0.0070micrometers), preferably 30 to 50angstroms. The width and height of thescratches 14 do not have to be uniform, although the scratches shouldnot be excessively wide or deep.

The texturing process results in an increase in the exposed surface areaof the metal coating(s) 13. The rougher surface reducesstiction/friction with the computer head and the substantiallycircumferential direction of the scratches enables betterdifferentiation between data tracks.

Although the illustration in FIG. 2 involved a substrate comprised ofmetal base coated with a thin film of metal or metal alloy, it is to beunderstood that the present invention also contemplates applying thetexturing method of the present invention to substrates made of glass orceramic material which have no metal or metal alloy thin film coatingformed on a surface thereof. Instead, the original surface(s) of theglass or ceramic substrate is directly subjected to the texturing methodof the present invention. The glass substrate material can be made of ahard amorphous glass material such as a fused mixture of the silicatesof the alkali and alkaline-earth or heavy metals. The ceramic substratematerial can be constituted by various hard materials made by shapingand then firing a nonmetallic mineral, such as clay, at a hightemperature. These ceramic materials include ceramic alloys, such assilicon nitride, silicon carbide, zirconia, and alumina.

Referring to FIG. 3, an abrasive article 30 of the present invention,includes a fibrous backing 32 having abrasive particles 36 and binder38. The binder 38 is a water soluble binder. Immediately before use ofthe abrasive article of the present invention in a texturing operation,the abrasive article should be exposed to a water or aqueous solventsource. This water exposure will penetrate into the fibrous backing 32and water soluble binder 38, softening the binder 38.

The fibrous backing is a porous, non-film backing. It can be either awoven or a non-woven substrate. Examples of non-wovens include knitted,lofty non-woven, and paper-like substrates. The substrate can be made ofstaple fibers, spun staple fibers, extruded fibers, etc. It has beenfound that a porous backing is greatly desired over a film backing (suchas polyester or polyester theraphthalate) due to its ability to removeswarf and debris from the texturing surface.

A porous backing is not completely sealed across its width and/orlength. For purposes of this invention, a "porous" backing can bedetermined by the following Porosity Test.

POROSITY TEST

The dry backing, prior to application of the abrasive coating, isinstalled on a Gurley Densitometer and the amount of time was measured(in seconds) to allow 100 cubic centimeters of air to pass through thebacking. This porosity test is well known in the textile industry.Briefly, the sample backing to be tested is secured at one end of ahollow metal cylinder of the densitometer. A piston that fits verytightly within the cylinder is then raised to allow exactly 100 cubiccentimeters of air at room temperature and pressure into the spacebetween the backing and piston. A timer is started at the exact momentwhen the piston begins to fall by the force of gravity toward thebacking. The time is measured for the 100 cubic centimeters of air topass through the backing. If the time is less than 100 seconds thebacking is considered foraminous or porous for purposes of thisinvention. Preferably, the time needed for the 100 cc of air to passthrough the backing will be less than 50 seconds for optimal porosity inthis invention. If the time is greater than 100 seconds, the backing isconsidered to be sealed, or nonporous.

The porous backing is a key aspect of this invention. During thetexturing process, swarf is generated; swarf is the term for thematerial abraded away or removed from the metal coating by the coatedabrasive article. As discussed above the texturing process is done in awet environment. Due to its porous nature, the swarf is able topenetrate into the backing and be removed from the abrading interface.If the swarf is not removed from the abrading interface, then the swarfcan become deposited on the high regions of the scratches in thetextured metal coating. This phenomena can lead to interference betweenthe rigid disk and the computer head. This interference can potentiallylead to a crash of the computer head.

The preferred fibrous backing for the present invention is a nonwovenbacking. A nonwoven can be described as a matrix of a randomdistribution of fibers. This matrix is usually formed by bonding thefibers together either autogeneously or by an adhesive. Examples ofnonwoven forms suitable for this invention include staple bonded, spunbonded melt blown, wet laid, needle punched or thermo-bonded. A nonwovenis typically porous, having a percentage of open space in the total webvolume of about 15% or more. Nonwovens are further described in "TheNonwoven Handbook" edited by Bernard M. Lichstein, published by theAssociation of the Nonwoven Fabrics Industry, New York, 1988.

The fibers in the porous fibrous backing can be natural or synthetic.Examples of fibers include: glass fibers, carbon fibers, mineral fibers,organic fibers, or ceramic fibers. The organic fibers can be natural orsynthetic. Examples of typical synthetic fibers include polyvinylalcohol fibers, rayon, polyethylene, polypropylene, nylon fibers,polyester fibers, phenolic fibers, aramid fibers, and combinations.Examples of natural fibers include cellulose, hemp, kapok, flax, sisal,jute, cotton, silk, manila, and combinations thereof.

Additionally, since the particle size of the abrasive particles isrelatively small, i.e., less than 25 micrometers, it is preferred thatthe fiber in the nonwoven backing be relatively fine. It is preferredthat the fiber diameter be less than about 30 micrometers, preferablyless than 20 micrometers.

The thickness of the fibrous porous backing generally ranges from 25 to800 micrometers, preferably between 100 to 375 micrometers. The weightof the backing generally ranges from 7 to 150 grams/square meter,preferably between 17 to 70 grams/square meter. It is within the scopeof this invention, to have only one layer of the backing or to havemultiple layers forming the backing. These multiple layers can rangefrom 1 to 10 nonwoven or woven layers, preferably between 2 to 5 layers.

The porous backing may further contain an adhesive to help bond thefibers together. However, the amount of the adhesive should not be sohigh that the backing is sealed. Examples of such adhesives includeresin emulsions like acrylonitrile butadiene emulsions, acrylicemulsions, butadiene emulsions, butadiene styrene emulsions, andcombinations thereof. Still other adhesives include thermosetting resinslike phenolic resins, acrylate resins, epoxy resins, urea-formaldehyderesins, urethane resins, and combinations thereof.

The backing of the invention may additionally comprise other additivesthat are well known in the art such as toughening materials, shapestabilizers, fillers, dyes, pigments, wetting agents, surfactants,coupling agents, antistatic agents, oils, flame retardants, ultravioletstabilizers, internal lubricants, antioxidants, and processing aids. Attimes it may be desirous that the fibrous porous backing has a film(such as polyester teraphthalate) laminated to the back non-abrasivecoated side to provide a barrier layer.

Bonded to at least one surface of the fibrous porous backing is theabrasive coating. The abrasive coating is formed from an abrasive slurrycomprising a plurality of abrasive particles distributed throughout awater soluble binder precursor.

Abrasive Particles

The abrasive particles typically have a particle size ranging from about0.05 to 10 micrometers, usually between about 0.1 to 5 micrometers andpreferably between 0.1 to 3 micrometers. This preferred particle sizerange is found to be most beneficial in generating an Ra value ofbetween about 30 to about 50 angstroms. If the abrasive particle size istoo large, then the resulting Ra value will be too large, i.e., greaterthan about 50. If the abrasive particle size is too small, then theresulting Ra value will be too small, i.e., less than about 30. It isalso preferred that the abrasive particle size distribution be verytightly controlled. The tight distribution is preferred to minimize theamount of wild scratches.

The abrasive particles have a Moh hardness of at least about 8, morepreferably above 9. Examples of such abrasive particles include fusedaluminum oxide, ceramic aluminum oxide, white fused aluminum oxide,heated treated aluminum oxide, silicon carbide, alumina zirconia,diamond, ceria, cubic boron nitride, garnet, and combinations thereof.The term abrasive particles also encompasses when single abrasiveparticles are bonded together to form an abrasive agglomerate.

Binder Precursor

The binder precursor is a water soluble polymer and remains watersoluble after the binder precursor is coated onto the fibrous backingand dried. Examples of such water soluble resins include polyvinylalcohol, starch, casein, vinyl acetate, polyethylene glycols,polypropylene glycols, gelatins and the like. Polyvinyl alcohol having anumber-average molecular weight from about 20,000 to 80,000 is thepreferred water soluble binder.

The binder solids content of the water soluble binder precursor coatingcomposition (before drying) is in the ratio range of from about 1:8 to1:4 binder solids:water, respectively, by weight, and preferably 1:5 to1:7 binder solids:water, respectively, by weight.

The binder precursor may further contain a non-water soluble polymer ornon-water soluble polymer precursor. However, the chemistry and theamount of these materials should be such that they do not adverselyaffect the erodibility of the abrasive coating.

The binder precursor is typically in a liquid state or an uncured ornon-polymerized state. After the abrasive slurry is coated onto theporous backing, the binder precursor is dried to a solid or nearly solidstate. Once the binder precursor is dried it is then converted into abinder and the abrasive slurry is converted into an abrasive coating.

The ratio in the abrasive coating of the abrasive particles:binder, byweight, is preferably between 7:1 to 3:1, preferably between 6:1 to 4:1,respectively.

Additives

The abrasive coating can further comprise optional additives, such as,for example, fillers (including grinding aids), fibers, lubricants,wetting agents, thixotropic materials, surfactants, pigments, dyes,antistatic agents, coupling agents, plasticizers, and suspending agents.The amounts of these materials are selected to provide the propertiesdesired. The use of these can affect the erodability of the abrasivecoating.

Examples of antistatic agents include graphite, carbon black, vanadiumoxide, humectants, and the like. These antistatic agents are disclosedin U.S. Pat. Nos. 5,061,294 (Harmer et al.); 5,137,542 (Buchanan etal.), and 5,203,884 (Buchanan et al.) incorporated herein by reference.

A coupling agent can provide an association bridge between the binderprecursor and the filler particles or abrasive particles. Examples ofcoupling agents include silanes, titanates, and zircoaluminates. Apreferred silane coupling agent is commercially available from UnionCarbide under the trade designation "A-174". The abrasive slurry(binder, additives, plus abrasive particles) preferably containsanywhere from about 0.01 to 3% by weight coupling agent based on totalweight of the abrasive slurry. The abrasive particles can be pretreatedwith the coupling agent, i.e., the coupling agent is applied directly tothe surface of the abrasive particle. Alternatively the coupling agentcan be mixed into the abrasive slurry.

An example of a suspending agent is an amorphous silica particle havinga surface area less than 150 square meters/gram that is commerciallyavailable from DeGussa Corp., under the trade name "OX-50".

Method of Making the Abrasive Article

The coated abrasive article of the invention can be made by thefollowing method. An abrasive slurry is prepared by mixing togetherabrasive particles, the binder precursor, and optional additives. Thepreferred mixing technique is a ball mill mixer with glass or ceramicmedia. The ball mill aids in preventing agglomeration of the abrasiveparticles. The abrasive slurry is coated onto at least one side of theporous fibrous backing by any conventional technique such as rollcoating, knife coating, die coating, spraying or curtain coating. Duringcoating, some of the abrasive slurry will penetrate into the backing,however, the abrasive slurry will not completely seal the backing. Next,the binder precursor is dried and the abrasive coating is formed. Thedrying conditions will depend upon the chemistry of the binderprecursor, but typical drying conditions include heating from about 80°to 150° C. for between 2 to 10 minutes.

The preferred coating technique is to use a coating technique whichproduces a uniform thickness, continuous or substantially continuouscoating on a surface of the backing, such as by using knurled rollcoating, roll coating, spray coating, die coating, curtain coating,extrusion knife coating, and the like. The thickness of the abrasivecoating will range between 3 to 25 micrometers, preferably between 5 to15 micrometers. It is also within the scope of this invention to coatthe abrasive slurry in a discontinuous pattern over the surfaceof thebacking, such as coating the abrasive coating as stripes or dots on thebacking.

FIG. 4 is a schematic representing a texturing apparatus 40 for use withthe method of the present invention. Although the texturing of only oneside of the substrate 42 is depicted, it is understood that both sidesof the substrate 42 can be typically textured simultaneously by separateabrasive articles or by the same abrasive article. Substrate 42 isgenerally between 50 to 200 millimeters in diameter, usually between 60to 150 mm. The substrate 42 is installed on a burnishing machine, suchas a EDC 800C HDF brand burnishing machine from the Exclusive DesignCompany (EDC), San Mateo, Calif. The machine spins the substrate 42between about 50 to 700 rpm, resulting in a surface speed on the disk ofbetween about 7.5 to about 440 meters/minute. The abrasive article 44 ofthe invention is preferably provided in a continuous roll form having awidth between 20 to 60 millimeters, preferably between 25 to 50 mm. Thecontinuous roll of the abrasive article 44 is unwound from one station46 to a second station 48. Shortly after station 46, abrasive article 44is exposed to a water spray (not shown) to soften the water solublebinder of the abrasive article construction. After the water spray, theabrasive article 44 contacts the metal coating on substrate 42 with theaid of a roller 50 as substrate 42 rotates. Roller 50 has a preferreddiameter of about 50 mm and is preferably constructed of an elastomericmaterial having a Shore A durometer of about 50. The texture cycle timeis generally about 30 seconds. The force between the abrasive article 44and the metal coating on substrate 42 is between 0.1 to 4 kg, preferablybetween 0.5 and 3 kg, for a contact length of 31.1 mm using a rubberroll having a diameter of 50 mm and a Shore A hardness value of 50. Ifthe pressure is too high, the resulting surface finish, Ra, will be toohigh, i.e., greater than about 7 nanometers (0.0070 micrometers). If thepressure is too low, then the scratch height will be low and the surfacefinish will be too low, i.e., less than about 2 nanometers (0.0020micrometers). The preferred method includes oscillating roller 50 in aradial direction relative to the substrate 42 during the texturingprocess. The radial oscillation ensures that the scratches formed by theabrasive article 44 are not concentric on the substrate 42, but are,instead, substantially circumferential with random crossings. Duringtexturing, the abrasive article 44 is also indexed at a controlled ratebetween stations 46 and 48 to provide a known and uniform abrading rateto the metal coating on substrate 42. The indexing speed of the abrasivearticle 44 is between 25 to 400 mm/minute, preferably between 50 to 250mm/min. The combination of the indexing abrasive article 44 and theoscillating roller 50 provides the random, substantiallycircumferentially spaced scratches desired. The rigid substrate 42 istypically cleaned to remove any debris or swarf after texturing. Aftercleaning, any conventional magnetic coating can be applied over thescratches. In a typical thin-film metal coated rigid magnetic mediadisk, a coating, such as chrome, is applied over a texturednickel/phosphorus coating. An additional coating of a magnetic materialis applied over the chrome coating, for example, a CoXY alloy, where Cois cobalt, X can be platinum or tantalum, and Y can be chrome or nickel.Finally a carbon coating is applied over the magnetic coating.

The abrasive article used in the method of the present invention canalso be used in abrading or polishing applications other than for thetexturing of thin film rigid disks. It is theorized that this abrasivearticle would be a good replacement for any application which uses looseabrasive slurries or fine grade fixed abrasives (such as lapping films).Examples of such applications include eyeglass lens polishing, stone andgem polishing, glass (stemware, TV tubes and screens), jewelry (gold,silver, etc.), and computer, video cassette recorder heads, and fiberoptic polishing.

Ra is a mathematical term used in the measuring of surface roughness. Rais the arithmetic average deviation of the absolute values of theroughness profile from the mean line or center line. The center linedivides profiles such that all areas above it are equal to all areasbelow it. Ra is usually reported in micrometers, angstroms, ornanometers.

EXAMPLES

The following non-limiting examples will further illustrate theinvention. All parts, percentages, ratios, etc., in the examples are byweight unless otherwise indicated. The following abbreviations are usedthroughout:

PEG: polyethylene glycol, commercially available from Union Carbideunder the trade designation Carbowax "600";

PVA1: polyvinyl alcohol binder, number-average molecular weight ofapproximately 25,000, commercially available from E. I. Du Pont deNemours & Co. under the trade designation "ELVANOL 51-05";

PVA2: polyvinyl alcohol binder, number-average molecular weight ofapproximately 80,000, commercially available from E. I. Du Pont deNemours & Co. under the trade designation "ELVANOL 50-40";

1-octanol: as surfactant, commercially available from Fisher Co.;

WAO: white aluminum oxide particles.

GENERAL METHOD FOR PREPARING THE ABRASIVE ARTICLE

The abrasive articles of the invention were prepared by the followinggeneral method. An abrasive slurry consisting of the listed materialswas prepared by blending with a ball mill until the slurry washomogenous. The slurry was then coated at a weight (i.e., applicationrate) of about 26 g/m² onto a surface of the fibrous backing using aknife coater. The coated backing was then dried in a conventional ovenat 120° C. for 3 minutes.

Texturing Procedure

The texturing test was performed on a rigid disk using the method of thepresent invention. A model 800C HDF Rigid Disk Burnisher, manufacturedby Exclusive Design Co., San Mateo, Calif., was used. The rigid disksubstrate was a nickel/phosphorus (NiP) plated aluminum disk (95 mmdiameter) rotated at 100 rpm. The abrasive article of the presentinvention was cut into a 51 mm wide abrasive strip having an extendedlength. Rolls of the abrasive strip were installed on a tape cassettethat had a supply reel with the unused abrasive article and a take-upreel with the used abrasive article. Two sets of abrasive tape cassetteswere tested. One cassette was used to texture the top surface of therigid disk, and the other cassette was used to texture the bottomsurface of the rigid disk. The feed rate of the abrasive tape andpressure between the tape and disk substrate were varied as stated inthe specific Examples. About 2.5 cm of the new abrasive tape was exposedto and contacted with an aqueous coolant mist prior to texturing. Theaqueous coolant mist was comprised of 5% by volume of "RECOOL 85"solution, which was a solution of sodium tolytriazole alkanolamine,commercially available from Mangill Chemical Co. In this regard,abrasive tape was prewet with the coolant to allow time for the binderto dissolve and release abrasive particles before the abrasive articlewas contacted with the disk. This prewetting of the abrasive article wasdone on the surface of abrasive coating of the abrasive tape at alocation 2.5 cm in front of and before the location of the abrasivearticle-disk interface, thus providing 10 to 30 seconds for the binderto dissolve before contact was initiated with the disk surface. Duringthe texturing process, a 5% by volume solution of the "RECOOL 85" in anaqueous coolant mist form was also dripped onto a cleaning fabric whichwas applied to the surface of the rigid disk to transfer the aqueouscoolant to the surface of the disk. Two cleaning tape cassettes (Type TJCleaning Tape, manufacture by Thomas E. West Co.) were also used in thistest. One cassette was used to clean the top surface of the rigid disk,and the other was used to clean the bottom surface of the rigid disk. Atthe surfaces of the rigid disk, the abrasive tapes and cleaning tapeswere passed over a Shore A 50 durometer elastomer roller which wasoscillated in a radial direction relative to the disk using a mechanicalvibrator with approximately 3 to 6 mm of travel. The endpoint of thetest was 20 seconds. The surface of the textured rigid disk was thenmeasured to determine the surface properties of each sample.

EXAMPLE 1

An abrasive article for Example 1 was prepared according to the GeneralMethod for Preparing the Abrasive Article. A porous woven twill cottonbacking, approximately 250 micrometers thick, was coated with WAO gradeWA 3000 (approximately 4 micrometer particle size or diameter).

EXAMPLE 2

An abrasive article for Example 2 was prepared the same as Example 1except WAO grade WA 4000 (approximately 3 micrometers in particle size)was used.

COMPARATIVE EXAMPLE A

Comparative Example A was an abrasive article comprising a polyesterteraphthalate backing with an abrasive coating containing a partiallycrosslinked polyester resin and 2 micrometer aluminum oxide. Thisproduct is commercially available from 3M Company, St. Paul, Minn. underthe trade designation "IMPERIAL TR3".

Electron photomicrographs confirmed that Examples 1 and 2 produced abetter line density and overall surface quality on textured rigidmagnetic disks (Ni-P surface film) under the same texturing conditions(interface pressure of 1.3 kg and a tape speed of 7.6 cm/minute) thanComparative Example A.

EXAMPLE 3

The abrasive article for Example 3 was prepared the same as Example 1except WAO having an average particle size of 3 micrometers was used.

EXAMPLE 4

The abrasive article for Example 4 was prepared the same as Example 3except WAO having an average particle size of 2 micrometers was used.

Examples 3 and 4 were tested at 5 different runs for texturing testparameters. Table 1 lists the following five run designations used atthe indicated different texturing parameters. The interface pressurevalue is given in kg and the tape speed is given in cm/min. Table 2reports the Ra and standard deviation of the Ra measurements (innanometers).

                  TABLE 1                                                         ______________________________________                                        Run         interface pressure                                                                         tape speed                                           ______________________________________                                        1           1.12         15.2                                                 2           1.12         5.1                                                  3           1.48         15.2                                                 4           1.48         5.1                                                  5           1.30         10.2                                                 ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        Example-Run     Ave. Ra  S.D. Ra                                              ______________________________________                                        3-1             3.29     0.31                                                 2-1             2.76     0.22                                                 3-2             4.15     0.28                                                 2-2             3.57     0.54                                                 3-3             3.60     0.37                                                 2-3             3.01     0.31                                                 3-4             3.99     0.45                                                 2-4             3.34     0.39                                                 3-5             3.95     0.46                                                 2-5             2.97     0.33                                                 ______________________________________                                    

EXAMPLE 5

The abrasive article of Example 5 was prepared according to the GeneralMethod for Preparing the Abrasive Article. A porous nonwoven cottonbacking, 178 micrometers thick, was coated with WAO grade having anaverage particle size of 2 micrometers.

COMPARATIVE EXAMPLE B

Comparative Example B was prepared according to the General Method forPreparing the Abrasive Article. A nonporous (sealed) polyesterteraphthalate film backing, 50 micrometers thick, was coated with WAOhaving an average particle size of 2 micrometers.

Example 5 and Comparative Example B were used to texture a rigid diskaccording. The above-described Texturing Procedure was used with thealteration that a 30 second cycle time was used. The following mean,median, and standard devaiation Ra data (in angstroms), summarized inTable 3, was generated from 20 and 19, respectively, measurements takenacross the textured surface of the disk.

                  TABLE 3                                                         ______________________________________                                                    Example 5                                                                             Comp. Ex. B                                               ______________________________________                                        Mean Ra       43.18     45.25                                                 Median Ra     43.6      45.3                                                  S. D. Ra      1.73      5.27                                                  ______________________________________                                    

The optimum disk finish would have consistent Ra measurements across thedisk with a low standard deviation. The areas of lower Ra on theComparative example B were visible as shiny bands. FIG. 5 shows a graphof Ra (angstroms) measured across the surface of the textured disk, fromouter diameter to inner diameter, with the locations of the measurementsindicated as the radial distance R from the inner diameter of the disk(i.e., the center hole). Both Table 3 and FIG. 5 show inconsistenttexture across the radius of the textured disk of Comparative Example B.The Ra values for Example 5 are indicated by the symbol "♦" while Ravalues for the Comparative Example B are indicated by the symbol "▪".The Figure shows the deviation across the disk when textured withComparative Example B, and Table 3 shows a significantly larger standarddeviation with the Comparative Example.

Various modifications and alterations of this invention will becomeapparent to those skilled in the art without departing from the scopeand spirit of this invention, and it should be understood that thisinvention is not to be unduly limited to the illustrative embodimentsset forth herein.

We claim:
 1. A method for texturing a surface of a metallic thin film ofa magnetic disk substrate comprising the steps of:(a) frictionallycontacting said surface of said metallic thin film with an abrasivearticle, wherein the abrasive article comprises an abrasive coatingincluding a water-soluble binder material and abrasive particles,wherein said abrasive coating is adhesively attached to a flexibleforaminous fibrous backing; and (b) abrading said surface of saidmetallic thin film by moving said metallic thin film and said abrasivearticle relative to one another with said abrasive coating being incontact with an aqueous liquid solution to form scratches in saidsurface of said metallic thin film, wherein said liquid solution is freeof abrasive particles before said contact with said abrasive coating. 2.The method of claim 1, wherein said foraminous fibrous backing comprisesa nonwoven fabric.
 3. The method of claim 2, wherein said nonwovenfabric comprises polyester fibers.
 4. The method of claim 1, whereinsaid water-soluble binder material is selected from the group consistingof polyvinyl alcohol, polyacrylamide, and polyethylene glycol.
 5. Themethod of claim 1, wherein said water-soluble binder material comprisespolyvinyl alcohol.
 6. The method of claim 1, wherein said metallic thinfilm comprises Ni-P.
 7. The method of claim 1, wherein said abrasivearticle comprises a sheet configuration frictionally conveyed over saidsurface of said metallic thin film by reel-to-reel means.
 8. The methodof claim 1, wherein said abrading step forms substantiallycircumferential scratches in said surface of said metallic thin filmwith an Ra of between about 20 angstroms to about 70 angstroms.
 9. Themethod of claim 1, wherein said abrading step forms substantiallycircumferential scratches in said surface of said metallic thin filmwith an Ra of between about 30 angstroms to about 50 angstroms.
 10. Themethod of claim 1, wherein said abrasive coating comprises said abrasiveparticles and said water-soluble binder in a weight ratio of 7:1 to 3:1,respectively.
 11. The method of claim 1, wherein said magnetic disksubstrate comprises a rigid metal base having opposing major surfaces,said metallic thin film formed on at least one of said major surfaces.12. The method of claim 1, wherein said rigid magnetic disk substratecomprises a circular shape and a central axis, and further wherein saidstep of abrading further comprises rotating said rigid substrate aboutsaid central axis to form substantially circumferential scratches. 13.The method of claim 12, wherein said step of rotating further comprisesforming said circumferential scratches at a speed of at least 7.5 metersper minute at an interface between said metallic thin film and saidabrasive article.
 14. The method of claim 1, wherein said abrasiveparticles have an average particle size of about 0.1 to about 5micrometers.
 15. The method of claim 1, wherein said abrasive particlescomprise aluminum oxide.
 16. The method of claim 1, wherein said step ofabrading further comprises oscillating said abrasive article in adirection substantially perpendicular to a direction of travel of saidrigid substrate during said abrading.
 17. The method of claim 1, furthercomprising the additional step after said abrading step of forming atleast one magnetic layer on said surface containing said scratches. 18.A method for texturing a surface of a metallic thin film of a magneticdisk substrate comprising the steps of:(a) providing an abrasive articlecomprising an abrasive coating including a water-soluble binder materialand abrasive particles, wherein said abrasive coating is adhesivelyattached to a flexible foraminous fibrous backing and comprises anexposed surface; (b) contacting said surface of said abrasive coatingwith an aqueous liquid solution, wherein said liquid solution is free ofabrasive particles before said contact with said abrasive coating; (c)frictionally contacting said surface of said metallic thin film withsaid abrasive article; and (d) abrading said surface of said metallicthin film by moving said metallic thin film and said abrasive articlerelative to one another to form scratches in said surface of saidmetallic thin film.
 19. The method of claim 18, wherein said abrasivecoating comprises a surface, and further comprising the step of wettingsaid surface of said abrasive coating with an aqueous solution prior tosaid step of frictionally contacting said surface of said metallic thinfilm with said abrasive article.